Candida albicans is the most common etiological agent of candidiasis, now the fourth leading cause of nosocomial infections. C. albicans is able to undergo reversible morphological transitions between unicellular yeast-like forms (blastospore) and filamentous forms (called hypha or pseudohypha depending on slight variations in the structure of the filament). These morphogenetic conversions are pivotal to its pathogenic potential. This belief is based upon the results obtained from a large number of virulence studies using C. albicans mutant strains unable to undergo this morphogenetic switch (i.e. delta cph1, delta efg1, delta tup1). However, all these strains are locked in one or other form so conclusions on the role of the morphogenetic transitions in virulence cannot be adequately addressed. Also, these mutant strains have been constructed using the URA-blaster technique with inherent problems that place serious questions on the genetic homogeneity between parental and disrupted strains. To overcome these problems, we have constructed a C. albicans strain which in which we have placed the recently discovered NRG1 gene under the control of a tetracycline-regulatable promoter. In C. albicans, Nrg1p is a DNA-binding protein that functions as a negative regulator of filamentation. Experiments confirmed the ability of doxycycline (DOX) to control the morphological transitions in this strain under a number of laboratory conditions. This gene expression system has also the advantage that it can be used in an animal host where expression of the gene of interest is regulated by simply adding DOX to the drinking water. The specific aims of this proposal are: i) to use this strain in a murine model of candidiasis in order to assess the role of NRG1 and the morphogenetic conversions in C. albicans virulence and ii) to identify downstream targets of Nrg1p under both planktonic and biofilm growing conditions.